All-trans retinoic acid (ATRA) triggers the differentiation of malignant pro-myelocytes and is now part of the standard remission induction/maintenance regimen for patients presenting with acute promyelocytic leukemia (APL). However, the effects of ATRA on other hematopoietic precursors is not as well characterized. We have recently accumulated evidence indicating that activated retinoic acid receptors enhance the generation and/or maintenance of relatively early multi- potent hematopoietic precursors including radioprotective cells, CFU-S, and short and long-term marrow repopulating stem cells. These observations suggest an unexpected and seemingly paradoxical dual role for retinoic acid receptors in regulating hematopoiesis, with one role being to enhance the commitment and terminal differentiation of relatively late hematopoietic progenitors but a second role being to stimulate the proliferation and perhaps self-renewal of primitive hematopoietic precursors/stem cells. It is this dual role of retinoic acid receptors that we wish to explore in the present application. The Specific Aim I studies will attempt to identify the in vitro culture conditions that optimize the ATRA effects on the generation/maintenance of hematopoietic stem cells in liquid suspension cultures. Specific Aim II will involve directly determining whether ATRA enhances the self- renewal of hematopoietic stem cells both in vitro and in vivo. Specific Aim III will address the question of whether ATRA enhances the efficiency of transducing hematopoietic stem cells with retroviral vectors. In Specific Aim IV we will attempt to identify the target genes that may be regulated by ATRA in primitive hematopoietic precursor with particular emphasis on Hox gene expression. These studies will provide insight into the roles of RA receptors in regulating primitive hematopoietic precursors and may also have direct clinical application to the ex vivo expansion and gene therapy of hematopoietic stem cells.